1. Field of the Invention
The instant disclosure relates to a calibration method and a calibration circuit; in particular, to a calibration method and a calibration circuit that calibrate mismatches between a first signal path and a second signal path of a receiver.
2. Description of Related Art
How to compensate mismatches between an in-phase signal path and a quadrature signal path of the receiver is rather important with regard to receivers used in the current communications field.
There are mainly two reasons why there will be mismatches between the in-phase signal path and the quadrature signal path. One is that, it is hard to have a perfect 90 degrees phase difference between the signal waves that are generated by a local oscillator and respectively input to an in-phase signal path and a quadrature signal path. In another aspect, the amplitudes of the signal waves that are generated by a local oscillator and respectively input to an in-phase signal path and a quadrature signal path are not equal. A phase difference and/or an amplitude difference mentioned above lead to mismatches between the in-phase signal path and the quadrature signal path. Even though the phase difference and/or the amplitude difference are minor, there is still an obvious interference affecting the whole communication system, which causes an abnormal quadrature demodulation and results in a loss of the received data and an increase of the bit error rate (BER).
Besides, even though the phase difference and amplitude difference can be calibrated, when the frequency of the input baseband signal varies or the baseband signal is input to a communication system that has a wider frequency band, there may still be mismatches between the in-phase signal path and the quadrature signal path of the receiver due to the frequency dependency, because mismatches between local oscillators and between mixers are not that much related to the frequency of the baseband signal. The above mismatches due to the frequency dependency are caused by differences between electrical characteristics of the corresponding circuit elements in the in-phase signal path and in the quadrature signal path, such as differences between electrical characteristics of the filter in the in-phase signal path and the filter in the quadrature signal path. In other words, the mismatches between the in-phase signal path and the quadrature signal path of the receiver may be generated because of certain defects in the electric element manufacturing process, which makes the electrical characteristics of the corresponding electric elements in the in-phase signal path and in the quadrature signal path not match perfectly, further making the signals passing through the in-phase signal path and the quadrature signal path different and generating an unexpected image interference. Accordingly, how to compensate mismatches between the in-phase signal path and the quadrature signal path of a receiver resulting from the frequency dependency is still a problem to be solved.